Symfony2 App with RESTful authentication, using FOSRestBundle and FOSUserBundle

I'm making REST API for my JS driven app.

During login, the login form is submitted via AJAX to url

/rest/login

of my API.

If the login is succesful, it returns 204

If it fails, it returns 401

While I have separated firewalls for the API and the app itself, they share the same context which should mean, that when user authenticates against the API, he's authenticated against the app too. So, when the server returns 204, page will reload and it should redirect user to the app, because he's now logged in.

/* first I need to somehow authenticate user
using normal authentication, that I've set up */
...

/* Then I need to return 204 or throw exception,
based on result.
This is done using FOSRestBundle and it's
listeners. */

if(!$succesful) {
throw new AuthenticationException();
}
}
}

I don't have a clue how to do that. Nothing I found in any documentation helped me a bit. I will be thankful for any suggestion that would point me in right direction.

EDIT: To further simplify, what I'm aiming for. I want my login to function exactly the same as normal form_login. I only want to change the response, that it sends back - instead of redirect I want 204 on success and 401 on failure.